In electron beam devices such as CRTs, an electron beam, or beams, are scanned across the inner surface of a display screen in a raster-like manner to activate the phosphor elements on the display screen in providing a video image. To maintain a video image of high resolution and definition, the electron beams must be maintained sharply focused on the display screen. The electron beams are generated, directed and focused on the display screen by means of a multi-grid electron gun. As the electron beams are scanned over the display screen, the distance from the center of the electron gun's main lens to the display screen, or the "throw distance," constantly changes. One common electron gun is known as a quadrupole, or QPF, electron gun having a prefocus lens formed of its G3, G4 and G5 grids and a main focus lens formed of its G5 and G6 grids. The QPF electron gun's G2 grid in its beam forming region (BFR) and its G4 grid are coupled together and charged by a common voltage source which reduces the number of voltage pins in the stem portion of the CRT's glass bulb. The dynamic QPF electron gun is characterized as having a variable quadrupole lens to compensate for the deflection yoke's astigmatism effect.
Referring to FIG. 1, there is shown a simplified longitudinal sectional view of a conventional QPF electron gun 10 which generates and directs three electron beams onto a display screen of a color CRT. QPF electron gun 10 includes three inline cathodes K which each direct electrons into a beam forming region (BFR) 12 comprised of a G1 control grid, a G2 screen grid, and a lower side of a G3 grid. QPF electron gun 10 further includes a symmetric prefocus lens 14 comprised of the upper side of the G3 grid, a G4 grid and the lower side of a G5 grid. The three electron beams are focused on a display screen of a color CRT (which is not shown in FIG. 1 for simplicity) by means of a main focus lens 16 comprised of the upper side of the G5 grid and a G6 grid. The G1 grid is typically maintained at zero voltage, while the G2 and G4 grids are typically coupled to a common voltage source Ec2 and the G3 and G5 grids are coupled to a common focus voltage source Ec3. The Ec2 voltage source maintains the G2 and G4 grids at a voltage in the range of 400-750V. The G6 grid is typically coupled to an accelerating, or anode, voltage source which is not shown in the figure for simplicity. Each of the three electron beams is directed through a plurality of aligned apertures in the various grids of electron gun 10 as the electrons proceed from the cathodes K toward the CRT's display screen.
Referring to FIG. 2, there is shown a simplified sectional view of a conventional prior art dynamic QPF electron gun 20. In the dynamic QPF electron gun 20 as in the previously described static QPF electron gun 10, the G2 and G4 grids are connected to and charged by a common voltage source Ec2. In the dynamic QPF electron gun 20, the G5 grid is divided into a G51 lower, a G52 middle, and a G53 upper grid. A fixed, or static, voltage source Ec3(S) is provided to and charges the G3 and G52 grids. A dynamic voltage is provided to and charges the G51 and G53 grids by means of a Ec3(D) variable voltage source. The dynamic voltage applied to the G51 and G53 grids varies as the electron beams scan the CRT's display screen in a to raster-like manner. As in the previously described static QPF electron gun 10, the dynamic QPF electron gun 20 also maintains the G2 and G4 grids at the same voltage, typically between 400-750V by connecting these grids to a common voltage source Ec2.
The unique feature of the QPF electron guns described above is that the G4 grid is connected to the G2 grid to permit formation of the G3-G4-G5 prefocus lens between the gun's beam forming region and its main focus lens without an extra voltage input pin in the stem portion of the CRT's glass envelope, or bulb (also not shown). Limiting the number of conducting pins extending through the stem portion of the CRT's glass envelope simplifies CRT design and reduces manufacturing costs. However, maintaining the G4 grid at the voltage of the G2 grid increases the strength of the electron gun's prefocus lens as well as the magnification of the electron beams which degrades video image quality.
The present invention addresses the aforementioned limitations of the prior art by reducing the strength of the electron gun's prefocus lens, resulting in reduced electron beam magnification and electron beam spot size on the CRT's display screen for improved video image quality.